1. Field of the Invention
The present invention relates to an image forming apparatus which continuously feeds sheets from a hopper and forms an image on each sheet.
2. Description of the Related Art
Image forming apparatuses, such as a copying machine, a printer and a facsimile, use a latent-image forming type recording apparatus like an electrophotographing apparatus due to a strong demand for printing on normal sheets of paper. Such an image forming apparatus first forms an electrostatic latent image on a photosensitive drum and then develops this electrostatic latent image, yielding a toner image. The image forming apparatus then transfers the toner image on the photosensitive drum onto a sheet of paper. In this transfer process, the sheet should be fed in synchronism with the timing of transferring the toner image on the sheet. If this timing is mismatched, the toner image undesirably shifts forward or backward on the sheet in the sheet feeding direction. Accordingly, a part of the intended print contents (toner image) may not be formed on the sheet.
In the transfer process, the sheet feeding speed should match with the rotational speed of the photosensitive drum (the moving speed of the toner image). To align the sheet with the toner image, the leading edge of the sheet should be parallel to the photosensitive drum 10 and the sheet should be fed in alignment with the toner image.
A sheet to be printed is first picked up from a hopper, is subjected to image transfer from the photosensitive drum, and is then received on a stacker through a fixing process. The position of the sheet during the feeding is likely to deviate from a desired position due to the slipping of the sheet at the time the sheet has been picked up from the hopper or sheet slipping during the feed. It is therefore difficult to align the sheet with the toner image on the photosensitive drum at the time of picking up the sheet. For instance, due to the material for sheets, a variation in the quality of sheets caused during the production and the ambient conditions such as humidity, the sheets may not have a uniform thickness or the amount of electrostatic charges on a sheet may not be uniform over the surface of the sheet, so that the feeding of sheets may vary from one sheet to another sheet.
To conduct a fast printing particularly, adequate measures should be taken since the distance from the hopper to the photosensitive drum is long and the positional deviation is accumulated as a sheet travels along the feeding path.
FIGS. 1, 2 and 3 are exemplary diagrams of prior art.
In an electrophotographing apparatus, for example, resist rollers R3 for adjusting the feeding timing are provided in the feeding path near a photosensitive drum 10 on which a toner image is to be formed, as shown in FIG. 1. Two sets (right and left sets) of feed rollers each set consisting of an upper roller and a lower roller are often used as the resist rollers R3. The resist rollers R3 rotate in the forward direction as well as in the reverse direction.
The conventional image forming operation will now be described with reference to FIGS. 2 and 3. Pickup rollers PR are activated to pick up sheets from a hopper 20. At the same time, feed rollers R1 and R2 are rotated. As a result, a sheet is picked up and fed by the feed rollers R1. When a sensor PS1 detects that the sheet has been picked up in this pickup operation, the rotation of the pickup rollers PR is stopped. An image drawing operation which an information light is impinged to the photosensitive drum 10, is started in synchronous with a detection on a front edge of a sheet by a detection sensor PS1.
When the leading edge of the sheet which is fed by the feed rollers R1 and R2 is detected by a sensor PS2 provided directly before the resist rollers R3, the rotation of the resist rollers R3 is reversed. Accordingly, the leading edge of the sheet fed by the feed rollers R1 and R2 abuts on the resist rollers R3 and stops temporarily. The feeding timing is therefore adjusted. This process also compensates for the skewing of the sheet.
Then, in accordance with the timing for alignment with the toner image on the photosensitive drum 10, the resist rollers R3 with which the leading edge of the sheet is in contact are rotated forward to start feeding the sheet again. After the sheet is accelerated to the same speed as the rotational speed of the photosensitive drum 10 until the sheet comes to the transfer position of the photosensitive drum 10, the toner image is transferred on the sheet.
Through the above processes, the toner image is transferred on the correct position on the sheet. That is, the sheet once picked up is stopped by the resist rollers R3 near the photosensitive drum 10 to compensate for the positional deviation of the sheet, and the sheet is then fed again in synchronism with the photosensitive drum 10 to be aligned with the toner image of the photosensitive drum 10.
The conventional scheme however has the following shortcomings. First, as sheets are picked up one after another by the pickup rollers in the continuous printing, the resist rollers are stopped after the trailing edge of a preceding sheet is fed out as shown in FIG. 2. The resist rollers should then rotate in the reverse direction to align the leading edge of the next sheet. The minimum time for starting feeding the next sheet is the time required for stopping and then reversing the rotation of the resist rollers. Accordingly, the distance d between the preceding sheet and the next sheet increases as shown in FIG. 2.
It is therefore inevitable that the exposure and developing positions on the photosensitive drum for the next sheet are apart from those for the preceding sheet. This reduces the effective performance of the image formation on the photosensitive drum, thus deteriorating the printing performance.
Secondly, a sheet should be accelerated so that its moving speed would match with the rotational speed of the photosensitive drum while the sheet moves from the resist rollers to the photosensitive drum. The feeding path should thus include a distance from the resist rollers to the photosensitive drum which is necessary for the acceleration of the sheet. If the feeding path is long, the positional deviation is accumulated accordingly as mentioned earlier, making it difficult to align the sheet with the toner image with higher precision.